The present invention relates to a device for controlling the attitude of an implement drawn by an agricultural machine.
Devices of the above-indicated type allow the attitude of the implement to be automatically controlled on the basis of the detection of the force which acts on the tool attachment so as to optimize the performance of the agricultural machine, that is to distribute the power of the machine correctly between the function of drawing or supporting the implement and the function of positioning the implement at a predetermined depth in the furrow being worked.
At present such devices have an electrical circuit constituted by four resistive elements fixed to a portion of a bar forming part of the tool attachment. These resistive elements are made of a material which has an electrical resistance value proportional to the expansion and contraction to which the elements themselves are subjected. These are connected in a Wheatstone bridge circuit in such a way that the mechanical stress due to the implement causes an imbalance of the bridge circuit to generate an electrical signal proportional to this imbalance and therefore to the mechanical stress. The electrical signal is then sent, via a signal amplifier circuit, to a central processing unit which forms part of a control system able to command raising and lowering of the implement, that is to say its attitude in relation to the desired working depth, the hardness of the ground and the power of the machine. The above-described devices have various disadvantages.
In particular it is to be noted that the bar is normally made of hardened steel so that significant difficulties must be overcome for the deposition of the resistive element in that the working stages for hardening the bar are contradictory with any further surface treatment such as the deposition of the resistive elements. Relative sliding can occur between the bar and the supports of the resistive elements, which causes the phenomenon of mechanical hysteresis and therefore a spurious electrical signal. It is to be noted that the resistive elements are deposited two on one face and two on the opposite face of the bar. This involves the production of several housing seats for the resistive elements the assembly of which takes place manually since there are no machine tools which can succeed in performing this assembly operation. Consequently errors in positioning the resistive elements can arise which cause errors in the electrical signal, which must therefore be compensated after assembly with a variation in the gain of the amplification circuit. Moreover, again after assembly has been completed, balancing of the Wheatstone bridge must be effected. Such devices are therefore difficult to assemble and once mounted must in general be recalibrated and adjusted, which involves high costs and extended times. Finally, it is to be noted that the electrical connections between the various resistive elements and between these and the amplification circuit are made with flying conductors, which involves a reduced protection of these, a lower durability due to wear (possible mechanical stresses and contacts with dust, oil and anything else which succeeds in entering into the protective casing) to which these are subject, the need for soldering to be effected during assembly, and in general a reduced reliability of the whole electrical installation.